Ring of invariants of an exponential map of a polynomial ring
Let $k$ be a field of arbitrary characteristic. We prove the following:
$\bullet$ Let $B=k^{[3]}$ and $δ\in \mathrm{EXP}(B)$ be non-trivial. If the plinth ideal $(\mathrm{pl} (δ))$ contains a quasi-basic element, then the ring of invariants ($B^δ$) is $k^{[2]}$.
$\bullet$ Let $B=R^{[n]}$, where $R$ is a $k$-domain and $δ\in \mathrm{EXP}_R(B)$ is a triangular exponential map. Then $B^δ$ is non-rigid.
We prove the following results for a $k$-domain $R$ when the characteristic of $k$ is…